1. Field of the Invention
This invention relates to electrical circuit devices, and more particularly to apparatus for selectively adjusting the operating characteristics of devices such as digital to analog converters
2. Description of the Prior Art
Due to slight variations in materials, environmental conditions, and the manufacturing process itself, it is impractical to manufacture many electronic circuit devices exactly to desired specifications in production quantities. Accordingly, for those devices in which manufacturing inaccuracies tend to go beyond acceptable limits, it is common to measure the device's operating characteristics at some point in the manufacturing process and adjust the circuit so as to eliminate or at least substantially reduce the measured inaccuracies. Examples in areas in which such adjustments are commonly made include trimming the output bit currents of a digital to analog converter (DAC), nulling an amplifier with the use of an external potentiometer, making frequency adjustments to active filters and voltage controlled oscillators, and modifying the logic functions of field programmable logic displays and read only memories.
In some of these applications the adjustment is made at an intermediate stage in the manufacturing process, and in others after manufacturing is completed. The problem with intermediate adjustments is that later assembly steps such as bonding and temperature sealing the device package can change the operating characteristics from their adjusted values. If no adjustments are made until the package is completely sealed however, there then arises a problem of access to the circuit. This problem has been resolved in the past by providing adjustment circuitry internal to the device package and allocating some of the package leads to this circuitry. While this effectively relieves the problem of introducing errors after adjustment is completed, in so doing it reduces the number of leads available for the basic device itself. In addition, since adjustment circuits are frequently digital in nature, the limited number of leads that can be devoted to the adjustment function restricts the range and resolution of the adjustment which can be achieved.
With respect to DAC's, various techniques are available for trimming their individual bit circuits in order to increase the accuracy of the output current signal produced by those bits. These methods include the employment of resistance networks or transistor current sources to modify the bit currents. The latter technique is described in copending patent application Ser. No. 768,328, entitled "Selectable Trimming Circuit for Use with a Digital to Analog Converter," filed on the same date as, and assigned to the assignee of the present application. It employs a plurality of current source transistors of varying magnitude for each trimmed bit, with a selectable shorting zener diode controlling the operation of each transistor. Each of the above techniques is characterized by two-terminal trimming devices which require a pair of signal inputs or connections to effect the desired trimming.
In the manufacture of integrated circuit DACs, the DAC circuit is first formed on a surface of an integrated circuit chip, lead wires are bonded on, and finally the chip is sealed to protect the circuit elements. Each of the last two steps may introduce modifications in the outputs of the bit circuits. For example, bonding on the lead wires creates mechanical stresses in the chip which can change resistance values, while the relatively high sealing temperature can introduce other variations into the circuit output. It is therefore highly desirable that trimming be accomplished after the circuit is completely packaged and final output characteristics established. This is particularly important in the higher accuracy converters which employ a large number of bits, since the required manufacturing accuracy generally varies exponentially with the number of bits. However, there are often few or no lead positions available for assignment specifically to trimming purposes. Accordingly, trimming has generally been performed before the packages are sealed and while the circuit elements are still directly accessible, leaving the devices open to the introduction of uncorrected errors during completion of the manufacturing process.